RE: [PATCH v4] vhost: support CPU copy for small packets

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Ma, WenwuX [mailto:wenwux.ma@intel.com]
> Sent: Tuesday, 27 September 2022 09.32
> 
> > From: Morten Brørup <mb@smartsharesystems.com>
> > Sent: 2022年9月7日 22:47
> >
> > > From: Wenwu Ma [mailto:wenwux.ma@intel.com]
> > > Sent: Monday, 29 August 2022 02.57
> > >
> > > Offloading small packets to DMA degrades throughput 10%~20%, and
> this
> > > is because DMA offloading is not free and DMA is not good at
> > > processing small packets. In addition, control plane packets are
> > > usually small, and assign those packets to DMA will significantly
> > > increase latency, which may cause timeout like TCP handshake
> packets.
> > > Therefore, this patch use CPU to perform small copies in vhost.
> > >
> > > Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
> > > ---
> >
> > [...]
> >
> > > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c index
> > > 35fa4670fd..cf796183a0 100644
> > > --- a/lib/vhost/virtio_net.c
> > > +++ b/lib/vhost/virtio_net.c
> > > @@ -26,6 +26,8 @@
> > >
> > >  #define MAX_BATCH_LEN 256
> > >
> > > +#define CPU_COPY_THRESHOLD_LEN 256
> >
> > This threshold may not be optimal for all CPU architectures and/or
> DMA
> > engines.
> >
> > Could you please provide a test application to compare the
> performance of
> > DMA copy with CPU rte_memcpy?
> >
> > The performance metric should be simple: How many cycles does the CPU
> > spend copying various packet sizes using each the two methods.
> >
> > You could provide test_dmadev_perf.c in addition to the existing
> > test_dmadev.c.
> > You can probably copy a some of the concepts and code from
> > test_memcpy_perf.c.
> > Alternatively, you might be able to add DMA copy to
> test_memcpy_perf.c.
> >
> > I'm sorry to push this on you - it should have been done as part of
> DMAdev
> > development already.
> >
> > -Morten
> 
> The test application has been supported by the following patch.
> 
> http://patchwork.dpdk.org/project/dpdk/patch/20220919113957.52127-1-
> cheng1.jiang@intel.com/
> 

Yes, thank you for reminding me.

Acked-by: Morten Brørup <mb@smartsharesystems.com>